Coupler for transmission line



Dec. 6, 1960 E. J. H. BUSSARD 2,963,665

COUPLER FOR TRANSMISSION LINE Filed Nov. 3, 1958 INVENTOR.

EMMERY J. H'. BUSSARD.

ATTOR YS.

United States Patent sOfiice COUPLER FOR TRANSMISSION LINE Emmery J. H. Bussard, Cincinnati, Ohio, assignor to Avco Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Filed Nov. 3, 1958, Ser. No. 771,542

6 Claims. (Cl. 333-9) This invention relates to electromagnetic coupling between branches of coaxial cables for the transmission of intelligencefor example, television, radio, and audio signalsbut more particularly to coupling devices by which pay-as-you-see subscribers are coupled into a distribution system and provided with special television or radio programs.

It is a primary object of the invention to provide a unitaryor multi-part coupler device of such construction that it may be quickly and easily installed by the cutting of a coaxial cable and then insertion of the device in place so that the characteristics of the transmission line are maintained.

A further object is to provide a coupling unit of such construction that a plurality of customers may be served from one unit, thus minimizing the number of breaks that have to be made in a coaxial cable in order to serve groups of subscribers in residential urban areas, for example.

One of the main objects of the invention is to provide a simple aperiodic coaxial cable transmission line coupler for bilateral transmission of signals having a frequency ratio in the order of l:1for example, 3 to 45 megacycles (mc.).

It is also an object of the invention to provide a coupler With multiple outlets to supply a large number of subscribers with service from a minimum of breaks in the transmission system.

Another object is to provide isolation in a coupler of such character that a great number of taps can be simultaneously operating without seriously degrading the distribution system.

The invention also contemplates the provision of auto matic compensation for undesired changes normally rehumidity.

A still further object is to provide a usable power transfer with core excitation which is conducive to linear transfer free of induced cross products.

The properties of magnetic core materials have been constantly improved until they become of utility at very high frequencies. Nevertheless, such materials cast into practical cores are relatively susceptible to temperature and humidity changes in addition to saturation effects throughout the core length Where an appreciable airgap or other high reluctance path exists in the magnetic loop.

Reference is parenthetically made, by way of usefully exploited background,to two commonly assigned patent applications now discussed.

In my copending patent application Serial No. 759,267, filed September 5, 195 8, entitled Automatic Ferrite Loop Antenna Loading,,and assigned to Avco Manufacturing Corporation, the assignee of the present application and invention, means are disclosed for the automatic compensation for changes in impedance of coredtransformers resulting from variable load reflected by such active de vices as transistors. One application described comprises a rod-type loop antenna with coupling winding sulting from variations in ambient temperature and Patented Dec. 6, 1960 driving a transistor base. Here the transistor is automatic-gain-controlled such that the load is varied in the inverse ratio directly with signal strength. This objectionable characteristic is removed by simultaneously introducing a proportional current into the primary winding. This alters the degree of saturation of the transformer core and compensates automatically for the unloading effected by change in the transistor gain.

In the copending patent application of Richard W. Bradmiller, SerialNo. 758,946, filed September 4, 1958, entitled Temperature Compensated Magnetic-Cored Inductor and also assigned to Avco Manufacturing Corporation, it is shown that temperature compensation can be accomplished automatically by two dissimilar opposed windings scientifically designed and positioned on a rod of magnetic core material. This accomplishes the temperature compensation electrically by varying the mu factor of the one winding faster than that of the other.

The copending applications of Bradmiller and Bussard referred to above showed that the effective values of L and Q characteristics of a magnetic cored inductor can be controlled and compensated automatically by novel principles. This invention, although generically independent of those two inventions, usefully employs their teachings as well as a novel structure herein disclosed, to provide an improved magnetic-cored transformer characterized by automatic compensation in all-weather service environment and operation.

This invention is particularly applicable to closed-circuit distribution of services such as television to community subscribers and the collection of metered data for such usage. The services may, for example, be transmitted over coaxial lines on one or more Wide-band channels, and the metered data would be telemetered back on distinct narrow-band channels. Each customer location involves bilateral transmissions through a coupler. That is to say, the subscriber equipment receives programs through means for coupling his equipment to a coaxial line in the distribution system. Further, each subscriber equipment sends metering information back to the dis tribution system. Such operation requires user isolation from the main distribution system to a degree sufficient to maintain a very low V.S.W.R. (voltage-standing-waveratio) and to reduce spurious interference to a tolerable level. Experience teaches that an attenuation of 54 decibels (db) across the coupler is desirable, while 40 db appears to be about the tolerable minimum under the most favorable circumstances. A 40 db condition is assumed for purposes of illustrating this invention, since it represents the condition of maximum power handling by the coupler. The discussion also assumes the use of core material which operates well into the V.H.F. rangee.g., 50 me. and higher.

It is desirable that the main coaxial cable and the subscribers coaxial line be matched in impedance and that the subscribers line be terminated in its characteristic impedance, in order to prevent undesired. reflections. The desired impedance match is accomplished with the greatest facility by providing a plurality of coupler secondaries. Four branch or subscriber circuits per coupler appear to be practical and to be consistent with both transformer and system equipment design and well adapted to logical subscriber layout plans. (Eight branches offer advantageous economies in densely populated areas.) Accordingly, four secondaries, supplying four branch or subscriber circuits, are provided in the preferred illustrative embodiment herein described in detail. This would mean that, in specific installations, the transformer design would be such that a 4:1 ratio of primary to secondary impedance existed, assuming the characteristic impedance on distribution and customer supply lines. It willbe understood that the transformer design needs to allow for the number of paralleled outletsi.e., in the case of two outlets the ratio would be 2:1; eight outlets, 8:1, etc.

In accordance with one aspect of the invention, there is provided a magnetic core 12 (Fig. l) of novel configuration and disposition which geometrically interlinks the inner conductor 11 of a coaxial line system and which has mounted thereon at least one secondary 17 adapted to establish a service connection to a subscriber. More narrowly viewed, the invention provides the combination of a pair of coaxial line sections 13, 14 and 15, 16 each having an inner conductor and an outer conductor, and a coupling transformer for maintaining continuity between such two sections while providing at least one branch circuit, such coupling transformer comprising an intermediate inner conductor 11 and an intermediate outer tubular conductor 10, each having two ends respectively adaptedto be joined in electrical and mechanical continuity with the counterpart elements in the aforementioned coaxial line sections, a loop-shaped core 12 of magnetic material geometrically eccentrically linking (see Fig. 2) said intermediate inner conductor, and a winding 17 on said core.

The above and other objects of the invention will be apparent to those of ordinary skill in the art to which the invention pertains from the following description and the accompanying drawings.

In the drawings:

Fig. 1 is a view in perspective of a preferred embodiment of my cored transformer coupler installed in a coaxial cable, the outer coaxial parts being shown broken away and only two windings being illustrated;

Fig. 2 is a view in section taken on line 2-2 of Fig. 1, with windings, excepting one, deleted for clarity in showing the core construction (the plane of this section is taken midway between the ends of core 12);

Fig. 3 is a view in section, taken on line 33 of Fig. 1, showing the transformer with the core laid out in an imaginary flat development;

Fig. 4 is a perspective of a practical adaptation of the cored transformer coupler of Fig. 1, showing a section of a coaxial branch transmission line connected into it; and

Fig. 5 is an end view of the Fig. 4 construction.

All section views are taken in the direction of the arrows as indicated.

In Fig. 1 a novel coupler in accordance with'the invention is shown installed in a coaxial cable transmission line, two sections of which are connected by the coupler and comprise outer tubular conductors 13 and and concentric inner tubular conductors 14 and 16. The coupler or coupling transformer maintains the electrical and mechanical characteristics and continuity of the transmission line while providing couplings to one or more branches or subscriber stations, the postulate being that the Fig. 1 transmission line sections are part of a system for the distribution of pay television programs or other high frequency signals.

The coupling transformer comprises the following principal members: an outer tubular conductor 10, an nner conductor 11, and a transformer core 12 mounted in eccentric linking relationship with the inner conductor 11. The core is positioned in place by a bracket 29, the bracket being suitably permanently connected to the outer conductor member 10 and having two legs press- .fitted as at 30 to an indentation in the core. The inner conductor 11 is held in embrace by a locator member 32, which properly spaces and registers the inner conductor 11 with reference to the core 12. The inner conductor 11 is a solid piece of conductive metal, formed with extensions at each end, such as 48, and conductor 11 is fitted into place by projecting these extensions tightly into the inner tubular conductors 14 and 16 (Fig. 3). It will be understood that the bracket 29 and the locator member 32 are made of suitable insulating dielectric material. The outer coaxial conductor 10 is similarly provided with extensions at each end, each closely fitting into the outer cable sections 13 and 15, as shown in representative manner at 47. The coaxial cable sections are provided with the usual dielectric spacer spirals or equivalent, as shown at 46 and 51.

The conductor members 10 and 11, being connected in a transmitting circuit which is terminated in a load, constitute the primary of a transformer. The core 12 of this transformer is made of ferromagnetic material having properties suitable for the frequency ranges to be transmitted. The core is generally of hollow cylindrical configuration, formed with a notch 31 (Fig. 1). In practical applications a notch width of approximately 40% of the core length and a notch depth approximating 50% of the inside radius of the transmission line have been found to be suitable. As best shown in Fig. 2, the core is eccentrically mounted with respect to inner conductor 11 and is formed as a generally ring-shaped continuum having a relatively large-radius arcuate portion (shown as the upper portion of the core in Fig. 2) and a relatively small-radius arcuate portion (the lower portion of the core as shown in Fig. 2), the relatively large-radius portion more early approaching the inner conductor member. That is to say, the core is preferably of an eggshaped section, to the end that it may be readily housed in such a manner as to maintain the continuity and normal transmission line characteristics of the cable in which the coupler is installed.

In the aforementioned copending patent application of Bradmiller, stability advantages were achieved by exploiting the varying rate of change in mu in a rod-type magnetic core. That principle is also exploited in this invention. The notching of the core, as at 31, increases the effective length of the core, and, as seen hereinbelow, the principle by which Bradmiller achieves enhanced stability and independence of ambient conditions such as temperature is usefully exploited.

It has been found that a suitable distance between the inner conductor 11 and the nearest portion of core 12 approximates one-third the inner radius of the coaxial cable, but this and all other dimensions herein given are offered for purposes of illustration and not of limitation.

The coupling transformer includes at least one customer service secondary and preferably a plurality of secondaries, four being herein shown for purposes of illustration.

Fig. 1 shows only two of the windings in the fourwinding embodiment herein disclosed, the remaining windings being deleted for pictorial simplicity in presenting the core construction. All four of the secondaries are illustrated in Fig. 3. A draftsmans privilege is also invoked in Fig. 3, in that the core 12 is represented in an imaginary fashion as it would appear if laid out flat with the unnotched portion thereof broken away at 52 and 53. This draftsmans liberty is taken for purposes of simplicity in explaining the winding layout.

As previously indicated, the illustrative embodiment herein shown comprises a plurality of secondaries 17, 20, 23, and 26. Each winding embodies, in series, a main winding and a compensating winding. For example, winding 17 comprises a main winding 18 completely wound around the core, and a compensating winding 19 wound about only one leg of the core. The winding section 19 is wound in opposition to, or out of phase with, winding section 18. Similarly, winding 20 comprises a main portion 21 wound completely around the core, and a bucking or compensating portion 22 wound about only one leg thereof. It will be observed that winding 20 in its entirety is wound in opposite phase relationship to winding 17.

As illustrated in Fig. 3, the windings 23 and 26 follow the same doctrines and structural arrangements as windings 17 and 20. That is to say, winding 23 comprises a main portion 24 and a bucking portion 25, and winding 23 as a whole-is out of phasewith winding 20. Similarly, winding 26 comprises amain portion 27 and a bucking portion 28. Again, winding 26, considered in entirety, is out of phase with winding 23.

Referring now specifically to winding 17, it should be observed that the bucking portion 19 has a relatively small inductance with respect to the main portion 18. This combination of winding sections 18 and 13 accomplishes automatic compensation for temperature changes, in the manner shown in the copending patent application of Bradmiller, referred to above. The opposing phase relationships between windings 17, 20, 23, and 26, taken as a whole, balance the magnetization load on the transformer. That is to say, it is desirable to alternate the direction of the secondary windings in the case of multiple service take-otfs, so that the influence of the windings on the core characteristics remains essentially balanced. This is desirable in order to avoid objectionable cross talk in the system.

The L and Q of specific systems are initially established for the particular installation by design considerations. Undesired changes which result from temperature and other environmental influences are automatically compensated for by the useful exploitation in my novel structure of several principles, including that taught in the aforementioned Bradmiller patent application. Additionally, as shown in the aforementioned copending patent application of Bussard, stability can be accomplished and environmental changes compensated for by applying a bias current to one or more of the secondary windings. The over-all result is that the effective values of the L and Q parameters are adjusted by the exploitation of such principles, which automatically compensates for changes resulting from the rigorous weather-exposed service environment in which the coupler of the present invention successfully operates.

One suitable application of the present invention postulates the transmission of a program via coaxial cable 13, 14, for example. This program is available to all customers on that line. Service to a specific customer is provided by tapping in my transformer comprising, inter alia, the members 10, 11, 12, 29, and 32. The secondary winding 17 is connected to the subscribers or customers c0- axial line (43, 44, shown in Fig. 4).

It will be understood that at the same time appropriate metering signals indicative of service usage are sent back to the distribution system by utilizing winding 17 (for example) as a primary for that purpose, it being coupled to a suitable transmitter at the subscribers location.

Referring now specifically to Figs. 4 and 5, a coupler unit in accordance with the invention is shown in position as installed, the diameter of member being exaggerated for purposes of exposition. The coupler unit is here formed as a hedgehog construction to permit easy access for connections of the subscribers cables. Four fittings, numbered 38, 40, 41, and 42, are shown, these being arranged after the fashion of cylinders in a V-8 engine, 38 and 40 being aligned and 42 and 41 being 90 degrees or so displaced therefrom. Fitting 38 is representative and is formed with exterior threads adapted to meet with complementary interior threads in the fitting 45 of the subscriber's line, which line consists of inner conductor 43 and outer conductor 44.

As shown in Fig. 2, fitting 38 is suitably permanently conductively and mechanically secured to outer conductor 10, and the inner conductor 37 of the fitting is connected to lead 36 of representative winding 17. The other lead 34 of winding 17 is brought out through a small opening in tubular conductor 10 and soldered in place at 35. The other windings are similarly connected to their fittings.

In practice a section of distribution line cable through which the program is transmitted is cut out and my novel coupler is fitted into place, as illustrated. Mechanical connections are suitably brazed or soldered where required, and the installation is completed with facility and the application of ordinary skills.

The coupler is basically a magnetic circuit, and it has been found that cross-products due to the heterodyning action of two or more signals can be avoided by operating on a linear portion of the magnetizing force magnetic flux characteristic curve. The current in the secondary or coupler tap (for example, winding 17) has a greater magnetizing effect on the magnetic core than the primary. The combined eifects of the program signals and the metering signals across the secondary winding determine, therefore, the linear operating state of the magnetic core characteristic and consequently minimize the amount of cross talk generated. In specific designs using more than one secondary, the effects of a plurality of secondaries are of course considered in determining the relative levels of the program signals and the metering signals.

The following additional representative parameters have been found suitable in a system in which a coupler in accordance with the invention is used:

Frequency band of television program signals being distributed 16-40 megacycles. Coaxial cable 13, 14 and 15, 70 ohm Spirafil /2" cable 16 with attenuation characteristics of .5 db/ maximum at 40 mc. Coaxial line 43, 44 70 ohm Spirafil cable with attenuation characteristics of .6 db/ 100' maximum at 40 mc. Transmission frequency of metering signals from subscriber location 3-5 megacycles.

While there has been shown and described what is at present considered to be the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the true scope of the invention as disclosed and as defined by the appended claims. For example, I do not intend to be limited to the specific parameters herein discussed. Further, the core component herein shown is believed to be per se inventive and adapted to various arrangements of windings in lieu of that herein shown. It is accordingly intended in the appended claims to cover all such changes and modifications as are within the proper scope of the invention.

I claim:

1. In a program-distribution system, the combination of a pair of coaxial line sections each having an inner conductor and an outer tubular conductor, and a coupling transformer for maintaining continuity between such two sections while providing at least one branch circuit, such coupling transformer comprising an intermediate inner conductor and an intermediate outer tubular conductor formed with an aperture therein, each having two ends respectively adapted to be joined in electrical and mechanical continuity with the counterpart elements in the aforementioned coaxial line sections, a loop-shaped core of magnetic material geometrically eccentrically linking and ringing said intermediate inner conductor, and a winding on said core, said winding having two leads, one of said leads being secured in contact with the outer conductor and the other lead projecting outwardly through said aperture.

2. Coupling means in accordance with claim 1 in which the core is formed as a ring-shaped continuum having a relatively large-radius arcuate portion and a relatively small-radius arcuate portion, the relatively large-radius portion more nearly approaching the inner conductor member, and means for securing said core in a fixed position relative to said inner conductor.

3. Coupling means in accordance with claim 1 in which the core is generally of egg-shaped section and is provided with a cut-out portion whereby the core comprises a plurality of spaced loops.

4. Coupling means in accordance with claim 3 in which said winding comprises a plurality of turns linking said core, together with a plurality of turns wound in opposition thereto and linking only one of said loop portions.

5. Coupling means in accordance with claim 4 and comprising a plurality of windings as defined in claim 4, one of said windings being wound in phase opposition to the other, whereby to limit saturation of said core, said outer conductor being formed with individual apertures for one lead of each winding, and each winding having a lead projecting through its associated aperture and a lead connected to said outer conductor.

' 6. Coupling means as in claim 5 in which the im- References Cited in the file of this patent UNITED STATES PATENTS 1,832,662 Schmutz Nov. 17, 1931 2,412,393 Ghosh Dec. 10, 1946 2,663,845 Koch Dec. 22, 1953 2,820,109 Dewitz Ian. 14, 1958 

